Dynamically vulcanized thermoplastic elastomers (thermoplastic vulcanizates) have a combination of both thermoplastic and elastic properties. Such thermoplastic vulcanizates are prepared by mixing and shearing a thermoplastic polymer, a vulcanizable rubber and a curing agent. The vulcanizable rubber is cured and is intimately and uniformly dispersed as a particulate phase within a continuous phase of the thermoplastic polymer.
Thermoplastic vulcanizates can be pressed and shaped into useful products using conventional plastic processing equipment. Thermoplastic vulcanizates can be made light in weight and attractive, with good durability, and can be reprocessed at the end of their product life to produce a new product. For these reasons, thermoplastic vulcanizates are widely used in industry, for example as auto parts, such as dashboards and bumpers, air ducts, seals and other under the hood applications; as gears and cogs, wheels and drive belts for machines; as cases and insulators for electronic devices; as fabric for carpets, clothes and bedding and as fillers for pillows and mattresses; and as expansion joints for construction.
In many applications where complete cross linking of the elastomer is desired, such as in seals and under the hood applications in the automotive industry, where oil and gasoline may cause swelling of the vulcanizate, limitations in prior art processing required the use of excessive amounts of curing agents. The excess curing agents, needed to insure complete cross linking may exude into the atmosphere, causing a health hazard or discomfort to those handling the material in addition to extra costs and inefficiencies. Additionally, the use of different curing agents, desirable for differentiated products suitable for different applications, typically requires modification of the screw elements, input ports, temperature profiles, and the like for optimal production. Any opportunity to decrease need for change in this manner would be of great benefit for manufacturers.
Thermoplastic vulcanizates can be prepared dynamically in Banbury mixers and other types of shearing mixers. Because of the advantages of a continuous process, such materials are often prepared in twin screw extruders. Twin screw extruders are available having screw diameters from 25 mm to about 380 mm. The larger extruders are the result of continuous progress toward increasing output. The extruders having the large screw diameters have a reduced surface area, relative to the output of the extruder, compared to extruders having screws with smaller diameters. Mixing in the larger extruders can be increased by increasing their length, but an increase in extruder length can increase material degradation. Consequently, the economies of scale offered by the larger extruders are not realized for all materials.
WO 03/031150 A1 teaches a method of using a twin screw extruder, which is said to reduce the occurrence of black specks in a dynamically vulcanized thermoplastic elastomer. The method describes the use of the extruder at 700 to 1100 RPM to solve the black speck problem. The publication teaches that the dynamically vulcanized thermoplastic elastomer may contain 65-95% by weight elastomer.
U.S. Pat. No. 4,594,390 teaches that dynamic vulcanization can take place in 20 to 60 seconds in a twin screw extruder at 100-500 RPM. The extrudate is said to be kneaded 1-30 times/second by each element, and is kneaded 200 to 1800 times for vulcanization to take place. The thermoplastic polymer and the elastomer are mixed in the first ⅓ of the extruder, catalyst is added at ⅓ of the extruder length from the feed throat, and dynamic vulcanization is carried out in the final ⅔ of the extruder length. Shear rates of at least 2000 sec−1 to 7500 sec−1 are described,
EP 0 547 843 B1 teaches at page 5, lines 7-10 that continuous single screw kneading extruders, twin screw kneading extruders, or kneading extruders with three screws or more, can be used for processing dynamically vulcanized thermoplastic elastomers. A twin screw extruder is described in EXAMPLES 1-4, and a super mixer is disclosed in EXAMPLE 4. The description does not indicate whether the screws in the extrusion are co-rotating, counter rotating or intermeshing.
U.S. Pat. No. 6,610,286 B1 describes a thermoplastic elastomer exhibiting scarce tendency of depositing gummy crust around the extrusion die upon extrusion. The patent alleges articles having superior oil resistance can be formed from the extrudate, and at column 4, recites a broad range of 5-70% by weight PP (polypropylene), where the total amount of PP and vulcanized oil-extended rubber in the composition is used for calculating the percentage. It is noted that in the examples, the least amount of PP shown in the compositions is 20% (25.9% excluding the oil extenders), and such materials are not isolated, but are intermediate materials that are projected to have been formed in processing.
WO 2004/009327 A1 describes a ring extruder having twelve intermeshing, co-rotating screws arranged around an inner core, and surrounded by a barrel. In their product brochure, 3+Extruder GmbH, Hoher Steg 10, 74348 Lauften, Germany, compares their Ring Extruder (RE) with twin screw extruders (TS), and claims better product quality with a higher throughput, and better cost efficiency. The large number of screw shafts are said to increase drive power and volume, and to provide easier and gentler mixing with less product degradation, and the barrel arrangement is said to provide efficient venting. The lengths of the barrels are based on the diameter of the screws designed for use in the barrel, and a feed barrel has a length of 3D (D is diameter), solid barrels have a length of 4D, and combination barrels have a length of 2D. Accordingly, in a ring extruder using mixing screws having a diameter of 30 mm, the feed barrel has a length of 90 mm, the solid barrel has a length of 120 mm, and the combination barrel has a length of 60 mm. The ring extruder is said to have advantages in that it can be made shorter because of the increased mixing capability of the increased number of screws, and equivalent output with smaller diameter screws provides advantages for heat transfer, mixing and degassing.